Add new page for demo selection.

Add new page for demo selection.

This patch adds all the components required to view and start an
OnlineGlom demo by clicking on the desired hyperlink. The user is
able to return to the demo selection page with the browser's back
button. I still need to modify the servlet to work with multiple
documents so all demo links will load the file defined in the
OnlineGlom.properties.

* .gitignore: Add .gwt which holds the error log for the GWT UiBuidler.
This is only useful during development so we don't need to save it.
* src/main/java/org/glom/web/client/ClientFactory.java: Add method to
get a reference to the DemoSelectionView.
* src/main/java/org/glom/web/client/ClientFactoryImpl.java: Implement
method to get a reference to the DemoSelectionView.
* src/main/java/org/glom/web/client/OnlineGlom.java: Change the
default view to DemoSelectionView.
* src/main/java/org/glom/web/client/OnlineGlomService.java: Add method
to get glom document titles for glom files in a hard-coded directory.
* src/main/java/org/glom/web/client/OnlineGlomServiceAsync.java: Add
method to get glom document titles for glom files in a hard-coded
directory.
* src/main/java/org/glom/web/client/activity/DemoSelectionActivity.java:
Presenter for DemoSelectionView.
* src/main/java/org/glom/web/client/mvp/AppActivityMapper.java: Update
for DemoSelectionView.
* src/main/java/org/glom/web/client/mvp/AppPlaceHistoryMapper.java:
Update for DemoSelectionView.
* src/main/java/org/glom/web/client/place/DemoSelectionPlace.java:
Basic 'Place' implementation for the DemoSelectionView.
* src/main/java/org/glom/web/client/ui/DemoSelectionView.java:
The interface for the DemoSelectionView.
* src/main/java/org/glom/web/client/ui/DemoSelectionViewImpl.java:
The implementation of the DemoSelectionView.
* src/main/java/org/glom/web/client/ui/DemoSelectionViewImpl.ui.xml:
The GWT UiBuilder xml file used in DemoSelectionViewImpl.
* src/main/java/org/glom/web/server/OnlineGlomServiceImpl.java: Add
implementation of method to get glom document titles for glom files
in a hard-coded directory.
* src/main/webapp/OnlineGlom.html: Remove link to CSS file as it's
on longer being used.
* src/main/webapp/glom.png: Glom logo.